The field of integrated photonics seeks to miniaturize optical components such as modulators, resonators, filters and waveguides, integrating them on a chip in the form of optical circuits. The fabrication of integrated photonic circuits through lithographic and other manufacturing methods promises to: (1) reduce cost and size, (2) increase complexity, and (3) improve overall performance of optical systems. Furthermore, high refractive index contrast (HIC) photonic circuits have the potential to further reduce component sizes and improve performance. Over the past decade, progress has been made in the field of HIC integrated photonics toward the development of practical and low-loss waveguides, high performance filters, resonators and modulators. However, numerous challenges need to be addressed to facilitate wide implementation of such integrated circuits. Among these challenges are the extreme sensitivity of HIC photonic circuits to fabrication uncertainties, and the sensitivities of HIC circuits to the environment.
Optical cavities are important in the field of integrated photonics because they form the building block for optical filters necessary to process and route data. However, as optical cavities shrink to smaller and smaller dimensions, in the context of HIC integrated photonics, dimensional sensitivities of the cavity's resonance frequency make it difficult to directly fabricate a HIC cavity to the designed optical frequency. For instance, the lithographic fabrication of identically patterned ring resonators generally results in a large range of resonance frequencies across the patterned wafer. However, dimensional variations, for instance, resulting from a slow variation in deposited layer thickness often result in a gradual drift of the cavity resonance frequency globally, while locally, the cavity frequencies are quite well matched (provided that the lithography is of high fidelity).
In addition to their extreme sensitivities to dimensional variations, HIC photonic circuits, such as those made from silicon, tend to be sensitive to their environment. For instance, since the refractive index of silicon changes rapidly with temperature, it may become challenging to stabilize the frequency of a cavity against thermal environmental variations.
Since a photonic microcavity, such as a microring, is generally of fixed dimensions, e.g., as a consequence of being “frozen” into a material on the surface of a lithographically patterned chip, tuning an integrated microcavity over a large wavelength range may be challenging. For example, the device path-length typically may not be changed sufficiently to appreciably tune the resonant wavelength. A more feasible approach to changing the resonance wavelength involves modifying the material that the waveguiding structure is comprised of, e.g., by heating or compressing it. Both types of perturbations generally result in a change of refractive index of the material, and, consequently, in a change of the effective index of the guided mode, which enables wavelength tuning. In this approach, the maximum tuning range is generally limited by the maximum thermal change that a heater can supply, or the maximum force that the material can sustain. In many cases, the maximum relative tuning of the cavity frequency that can be achieved is about 1%, which is far less than what is commonly achieved in bulk-component, free-space tunable fabry-perot filters.
An alternative way to tune microcavities is to change the effective index of the guided mode through optomechanical means rather than by changing material properties. This method enables cavity tunings of about 10%, as a change in waveguide geometry may be designed to result in a much larger change in effective index than material perturbations can generally afford. However, optomechanical cavity tuning generally requires precise control of the relative motion of a perturbing structure and the waveguide, often on a picometer scale, to stabilize the cavity frequency to a useful degree.
Accordingly, there is a need for tunable, stabilized HIC photonic devices with increased tolerance for fabrication errors.